On several occasions, spacecraft landers or probes attempting to land onto a planet surface have crashed with a total loss of the equipment. This has been especially true for missions to Mars. The distance between Earth and Mars results in a communications time delay of about 20 minutes in each direction, such that the lander must act autonomously, based solely on pre-loaded software and parameters. Every potential failure mode must be anticipated by the design team and appropriate responses determined and programmed in advance.
Landing a large mass on Mars is particularly challenging. The atmosphere of Mars is too thin for aerobraking and parachutes alone to be effective, while remaining thick enough to create stability and impingement problems when decelerating with retro-rockets. Parachutes, thrusters, and airbags, either on their own or in combination, which were employed for the successful landings of the Sojourner, Spirit and Opportunity rovers, all less than 200 kg, don't work for landing payloads of one metric ton or more on Mars. Viking 1 and 2 landers (572 kg) and the Curiosity rover (899 kg) have used a combination of heat shields and thrusters. The Curiosity rover finished its descent using a rocket-powered sky crane system, rather than the airbags used with lighter rovers. Proposed craft for future manned landings upon Mars have an overall payload of 100 metric tons. Such a vehicle is too massive to rely upon parachutes and/or a “sky crane” tethered system to descend to the Martian surface. Supersonic retro-propulsion using thrust from large rocket engines are expected to do the job.
In December 1999, a landing attempt by NASA of the Mars Polar Lander suffered from a premature shutdown of its descent engine just prior to the lander touching the surface, causing it to strike the planet at a high velocity (about 80 kph). In that case, a software error incorrectly interpreted vibrations from deployment of the stowed legs as a surface touchdown while the lander was still 40 meters above the surface. In the same mission, two Deep Space 2 hard penetrator probes failed to survive a planned impact. More recently, the Schiaparelli Entry, Descent and Landing Demonstrator Module (EDM) portion of the ExoMars project, a joint mission of the European Space Agency (ESA) and the Russian space agency Roscosmos, crashed on 19 Oct. 2016. Early indications are that the deployed parachute was ejected too soon, and rocket thrusters shut off prematurely, firing for only 3 seconds instead of the expected 30 seconds, such that the lander impacted the Martian surface at near terminal velocity (about 300 kph). Although the lander crashed, ESA officials nevertheless declared Schiaparelli a partial success because it had fulfilled its primary function of testing the landing system for the ExoMars 2020 surface platform and returning telemetry data during its descent.
Overall, the Mars lander missions to date have achieved success only about half the time. More launches for Mars lander missions are planned for April/May 2018, the next favorable launch window, and again for July-September 2020. Unmanned missions to Phobos (a Mars moon), Europa (a moon of Jupiter), and other space bodies (such as asteroids) are planned.
The United States has committed NASA to a long-term goal of human spaceflight and exploration beyond low-earth orbit, including crewed missions toward eventually achieving the extension of human habitation on another celestial body (e.g., the Moon, Mars). There have also been private proposals to send manned space vehicles to Mars by 2025. One such development project is the Mars Colonial Transporter by the private U.S. company SpaceX with plans for a first launch in 2022 followed by flights with passengers in 2024.
This research and development activity is expected to proceed in several general stages, beginning with an Earth-reliant stage with research and testing on the ISS of concepts and systems that could enable deep space, long-duration crewed missions, followed by a proving ground stage in cis-lunar space to test and validate complex operations and components before moving on to largely Earth-independent stages. An Earth-independent stage is planned for Mars orbit, tentatively including a phase for perfecting a Mars lander suitable for human-scale landings, an ascent vehicle to reach orbit from the Mars surface, and a possible Mars orbital transport vehicle or “taxi” for transportation between a long-duration Martian surface habitat and the Mars' moons or low Mars orbit.
The negative impact of any lander failure in such a manned mission upon public opinion and investor confidence would likely be an enormous setback to space exploration. Even the landings that inadvertently crashed were timed, as best as possible, for near ideal surface conditions. Eventually, however, landings will be required during all seasons, even where the landing conditions are less than optimal. For example, on Mars, during certain seasons when CO2 dry ice in one or the other polar ice cap is rapidly sublimating into the Martian atmosphere, surface winds are created that can adversely impact upon landing craft descent and drift, adding to landing risk or missing the targeted landing site by many kilometers. At such times, lander reliability is particularly important. Hence, space agencies and private space companies are continuing to seek ever more reliable landing systems to minimize the possibility of such an occurrence.
Once on Mars, transport from one base or site to another would be complicated by Martian surface conditions. Due to the thin atmosphere of Mars (with a pressure of merely 0.6% that on Earth), air transport by way of airplane or helicopter are impossible, even with Mars' lower gravity (38% of that on Earth), because of insufficient lift. This leaves fewer forms of transportation on Mars (e.g., rovers) or various launch and re-entry schemes. The latter could be rocket-powered or ballistic, and orbital or sub-orbital. Due to the current failure rate of lander missions to date, landing systems would need to be significantly improved before launch-and-reentry type transport schemes could become commonplace.
Several projects have explored the possibility of nuclear spacecraft propulsion. The first of these was Project Orion from 1958-1963 built upon general proposals in the 1940s by Stanislaw Ulam and others, in which external atomic detonations would form the basis for a nuclear pulse drive. Later, between 1973 and 1978, Project Daedalus of the British Interplanetary Society considered a design using inertial confinement fusion triggered by electron beams directed against fuel pellets in a reaction chamber. From 1987 to 1988, Project Longshot by NASA in collaboration with the US Naval Academy developed a fusion engine concept also using inertial confinement fuel pellets but this time ignited using a number of lasers. All of these projects have focused on its use for propulsion in interplanetary space. Naturally, these last two projects depend upon successfully achieving nuclear fusion. Additionally, the atomic detonations of the type proposed in the Orion project are wholly unsuitable for use in landing a spacecraft on a planetary surface because of dangers inherent in high-energy detonations near the landing site and from potential contamination of the site itself.
Muon-catalyzed fusion was observed by chance in late 1956 by Luis Alvarez and colleagues during evaluation of liquid-hydrogen bubble chamber images as part of accelerator-based particle decay studies. These were rare proton-deuteron fusion events that only occurred because of the natural presence of a tiny amount of deuterium (about one part per 6400) in the liquid hydrogen. It was quickly recognized that fusion many orders of magnitude larger would occur with either pure deuterium or a deuterium-tritium mixture. However, John D. Jackson (Lawrence Berkeley Laboratory and Prof. Emeritus of Physics, Univ. of California, Berkeley) correctly noted that for useful power production there would need to be an energetically cheap way of producing muons. The energy expense of generating muons artificially in particle accelerators combined with their short lifetimes has limited its viability as an earth-based fusion source, since it falls short of break-even potential.
Another controlled fusion technique is particle-target fusion which comes from accelerating a particle to sufficient energy to overcome the Coulomb barrier and interact with target nuclei. To date, proposals in this area depend upon using some kind of particle accelerator. Although some fusion events can be observed with as little as 10 KeV acceleration, fusion cross-sections are sufficiently low that accelerator-based particle-target fusion are inefficient and fall short of break-even potential.
It is known that cosmic rays are abundant in interplanetary space. Cosmic rays are mainly high-energy protons (with some high-energy helium nuclei as well) with kinetic energies in excess of 300 MeV. Most cosmic rays have GeV energy levels, although some extremely energetic ones can exceed 1018 eV. FIG. 8 shows cosmic ray flux distribution at the Earth's surface after significant absorption by Earth's atmosphere. In near-Earth space, the alpha magnetic spectrometer (AMS-02) instrument aboard the International Space Station since 2011 has recorded an average of 45 million fast cosmic ray particles daily (approx. 500 per second within that instrument's effective acceptance area and measurement energy range). The overall flux of galactic cosmic ray protons (above earth's atmosphere) can range from a minimum of 1200 m−2s−1sr−1 to as much as twice that amount. (The flux of galactic cosmic rays entering our solar system, while generally steady, has been observed to vary by a factor of about 2 over an 11-year cycle according to the magnetic strength of the heliosphere.) In regions that are outside of Earth's protective magnetic field (e.g. in interplanetary space), the cosmic ray flux is expected to be several orders of magnitude greater. As measured by the Martian Radiation Experiment (MARIE) aboard the Mars Odyssey spacecraft, average in-orbit cosmic ray doses were about 400-500 mSv per year, which is an order of magnitude higher than on Earth.
Cosmic rays are known to generate abundant muons from the decay of cosmic rays passing through Earth's atmosphere. Cosmic rays lose energy upon collisions with atmospheric dust, and to a lesser extent atoms or molecules, generating elementary particles, including pions and then muons, usually within a penetration distance of a few cm. Typically, hundreds of muons are generated per cosmic ray particle from successive collisions. Near sea level on Earth, the flux of muons generated by the cosmic rays' interaction by the atmosphere averages about 70 m−2s−1sr−1. The muon flux is even higher in the upper atmosphere. These relatively low flux levels on Earth reflect the fact that both Earth's atmosphere and geomagnetic field substantially shields our planet from cosmic ray radiation. Mars is a different story, having very little atmosphere (only 0.6% of Earth's pressure) and no magnetic field, so that muon generation at Mars' surface is expected to be very much higher than on Earth's surface.